Shear flow of non-Brownian rod-sphere mixtures near jamming.

We use the discrete element method, taking particle contact and hydrodynamic lubrication into account, to unveil the shear rheology of suspensions of frictionless non-Brownian rods in the dense packing fraction regime. We find that, analogously to the random close packing volume fraction, the shear-driven jamming point of this system varies in a nonmonotonic fashion as a function of the rod aspect ratio. The latter strongly influences how the addition of rodlike particles affects the rheological response of a suspension of frictionless non-Brownian spheres to an external shear flow. At fixed values of the total (rods plus spheres) packing fraction, the viscosity of the suspension is reduced by the addition of "short"(≤2) rods but is instead increased by the addition of "long"(≥2) rods. A mechanistic interpretation is provided in terms of packing and excluded-volume arguments.

Women in dental education and their desire to lead: A follow-up.

The aim of this study was to provide a follow-up to the 2015 national survey of women in dental education to re-examine factors and perceived barriers to pursuing administrative and leadership roles. At the beginning of 2023, a survey was administered to full-time women faculty in predoctoral dental programs in the United States. The survey instrument employed a structured format along with open-ended questions to capture qualitative data. The overall response rate was 26% (527/1994) and included respondents from fifty-five of the sixty-seven dental schools contacted. Half of respondents indicated holding an administrative position and 20% reported administration as their primary appointment. There was a significant increase in the number of women holding a leadership role in 2023 (76%) compared to 2015 (53%). Those seeking a leadership position in 2023 were less likely to have participated in leadership training (p<0.01) and less likely to have previously sought an administrative position (p>0.01). Women in leadership and administrative roles were more likely to indicate receiving extra compensation for administrative duties (p<0.01) and a desire for higher rank (p = 0.04) compared to 2015, but perceptions persist among the majority of respondents (71%) that women are not being compensated equally to their male counterparts. Respondents highlighted the negative impact of the COVID-19 pandemic on career advancement and the positive culture changes that have contributed to greater workplace flexibility. Overall, respondents sent a resounding message that women in dental education need structural change to achieve success: pay equity, mentorship, and inclusive and supportive work environments.

Scaling Description of Frictionless Dense Suspensions under Inhomogeneous Flow.

Predicting the rheology of dense suspensions under inhomogeneous flow is crucial in many industrial and geophysical applications, yet the conventional "µ(J)" framework is limited to homogeneous conditions in which the shear rate and solids fraction are spatially invariant. To address this shortcoming, we use particle-based simulations of frictionless dense suspensions to derive new constitutive laws that unify the rheological response under both homogeneous and inhomogeneous conditions. By defining a new dimensionless number associated with particle velocity fluctuations and combining it with the viscous number, the macroscopic friction, and the solids fraction, we obtain scaling relations that collapse data from homogeneous and inhomogeneous simulations. The relations allow prediction of the steady state velocity, stress, and volume fraction fields using only knowledge of the applied driving force.

Impact of granular inclusions on the phase behavior of colloidal gels.

Colloidal gels formed from small attractive particles are commonly used in formulations to keep larger components in suspension. Despite extensive work characterising unfilled gels, little is known about how the larger inclusions alter the phase behavior and microstructure of the colloidal system. Here we use numerical simulations to examine how larger 'granular' particles can alter the gel transition phase boundaries. We find two distinct regimes depending on both the filler size and native gel structure: a 'passive' regime where the filler fits into already-present voids, giving little change in the transition, and an 'active' regime where the filler no longer fits in these voids and instead perturbs the native structure. In this second regime the phase boundary is controlled by an effective colloidal volume fraction given by the available free volume.

Bulk rheology of sticky DNA-functionalized emulsions.

We measure by experiment and particle-based simulation the rheology of concentrated, non-Brownian droplet emulsions functionalized with surface-bound single-stranded (ss), "sticky," DNA. In the absence of ssDNA, the emulsion viscosity increases with the dispersed phase volume fraction ϕ, before passing through a liquid-solid transition at a critical ϕ_{c} related to random close packing. Introducing ssDNA leads to a liquid-solid transition at ϕ<ϕ_{c}, the onset being set by the droplet valency N and the ssDNA concentration (or simulated binding strength ε). Using insight from simulation, we identify three key behaviors: (i) jammed suspensions (ϕ>ϕ_{c}≈0.64) show weak effects of functionalization, with elastic rheology instead governed by droplet stiffness; (ii) suspensions with ϕ<ϕ_{c} and N=1, 2 always exhibit viscous rheology, regardless of functionalization; and (iii) for ϕ<ϕ_{c} and N>3, functionalization leads to a controllable viscous-elastic transition. We present state diagrams showing the range of rheological tuning attainable by these means.

Expert consensus on Didactic Clinical Skills Development for orthodontic curricula.

PURPOSE/OBJECTIVE: Competence is expected of each beginning dentist and orthodontist. However, the broad definition of competence presents a challenge to academic programs in identifying the level of cognition for students to achieve competence. This study aimed to determine the Didactic Clinical Skills Development curriculum content and competency in predoctoral and advanced education orthodontic programs. METHOD: A modified Delphi method with a consensus threshold of 70% was employed using an expert panel of academic orthodontists. RESULTS: Round One (n = 26) identified that all topics proposed by a focus group were necessary, except for predoctoral Appliances, which was at 65%. Round Two (n = 23) included subtopics of Appliances to confirm the lack of consensus, plus subtopics of all the other topics and the level of cognition required for each subtopic. The expert panel reached a consensus that all 24 subtopics, hence all topics, were necessary. In Round Three, subtopic responses in Round Two were assigned a value between 1 (remember) and 6 (create) to generate a hierarchical level-of-learning scale. Mean values were calculated for each subtopic response. For all subtopics, the mean level of cognition for predoctoral education was at understand; for advanced education, it was at evaluate. CONCLUSION: This consensus suggests that, to be deemed competent, beginning dentists must learn these topics and subtopics in the cognitive domain of understand, and beginning orthodontists in the cognitive domain of evaluate. This study showed an expert consensus on Didactic Clinical Skills Development orthodontic curriculum content and a panorama of educational objectives that could be used as a template for curriculum design.

Reliability of a grading rubric designed to evaluate the reflective ability of predoctoral dental students.

INTRODUCTION: Reflection and self-assessment are critical skills for healthcare providers. Identification of gaps in knowledge, skills and attitudes, along with the ability to critically think and problem solve to fill gaps, is the ultimate outcome for lifelong learning. The aims of this study were to (a) refine an instrument used for measuring reflective ability, and conduct comprehensive reliability testing, and (b) describe a process for rater calibration. MATERIALS AND METHODS: Students develop e-portfolios over a four-year span with assignments that require reflection and self-assessment. The final piece of the portfolio includes a global reflection written the last semester of the programme. Three faculty raters independently evaluated 106 dental students' global reflections using the revised grading rubric. An intraclass correlation coefficient measured the level of agreement between the three raters. RESULTS: Analysis of the 318 faculty ratings (106/rater) resulted in an intraclass correlation of .708. Based on a 5-point grading scale (0 = does not respond to the assignment to 5 = reconstructing), the ratings of the 106 global reflections ranged from 1.3 to 5.0 (M = 3.1, SD =0.66). DISCUSSION: This study provides confidence in the reliability of a grading rubric designed to assess reflective ability, along with suggestions for calibration. An overall mean of 3.1 (Level 3 = relating-includes evidence of lessons learned) illustrates the complexity of teaching reflection and self-assessment. CONCLUSION: Use of a reliable grading rubric for assessing reflective writing could assist schools interested in incorporating reflection and self-assessment into the curriculum, ultimately supporting lifelong and enhanced health care.

Nonequilibrium master kinetic equation modeling of colloidal gelation.

We present a detailed study of the kinetic cluster growth process during gelation of weakly attractive colloidal particles by means of experiments on critical Casimir attractive colloidal systems, simulations, and analytical theory. In the experiments and simulations, we follow the mean coordination number of the particles during the growth of clusters to identify an attractive-strength independent cluster evolution as a function of mean coordination number. We relate this cluster evolution to the kinetic attachment and detachment rates of particles and particle clusters. We find that single-particle detachment dominates in the relevant weak attractive-strength regime, while association rates are almost independent of the cluster size. Using the limit of single-particle dissociation and size-independent association rates, we solve the master kinetic equation of cluster growth analytically to predict power-law cluster mass distributions with exponents -3/2 and -5/2 before and after gelation, respectively, which are consistent with the experimental and simulation data. These results suggest that the observed critical Casimir-induced gelation is a second-order nonequilibrium phase transition (with broken detailed balance). Consistent with this scenario, the size of the largest cluster is observed to diverge with power-law exponent according to three-dimensional percolation on approaching the critical mean coordination number.

Scholarly Inquiry and Research: An assessment of graduate dental hygiene schools' requirements.

Purpose: Scholarly inquiry and research are core competencies for graduate dental hygiene education as defined by American Dental Education Association (ADEA). The purpose of this study was to examine how graduate dental hygiene programs in the United States (US) are meeting these competencies.Methods: The study sample consisted of the graduate programs in the US that award a terminal degree specific to dental hygiene (n=14). Graduate program directors were invited via email to participate in an electronic survey. The survey questions were developed based on the ADEA graduate dental hygiene education competency for scholarly inquiry and research. Descriptive statistics including frequencies and percentages were used to analyze the data. Exploration of relationships between variables were conducted using correlational analyses and t-tests.Results: A response rate of 71% was achieved (n=10). There was a significant difference in the minimum number of scholarly activity requirements between programs with lower student enrollments (M=4.43, SD=1.61) versus those with higher enrollments (M=2.00, SD=0; t(8)=2.51, p=.036). A negative correlation was found between the submission of a manuscript to a peer reviewed journal and the number of students accepted per year in the graduate program (r (10)= -.655, p <.05), indicating that students graduating from programs with larger enrollments were less likely to submit their scholarly work for publication.Conclusions: All program directors reported requiring students to participate in at least one scholarly activity as defined in the ADEA Core Competencies for Graduate Dental Hygiene Education. Program size was the biggest variable in relationship to the number of scholarly requirements. Schools with smaller enrollments required their students to participate in over twice the number of scholarly activities as compared to programs with larger enrollments. More research is needed to evaluate how graduate level dental hygiene programs are meeting the ADEA competencies.

Stokes at 200 (part 2).

We present the second half of the papers from the Stokes200 symposium celebrating the bicentenary of George Gabriel Stokes. This article is part of the theme issue 'Stokes at 200 (part 2)'.

Nonequilibrium continuous phase transition in colloidal gelation with short-range attraction.

The dynamical arrest of attractive colloidal particles into out-of-equilibrium structures, known as gelation, is central to biophysics, materials science, nanotechnology, and food and cosmetic applications, but a complete understanding is lacking. In particular, for intermediate particle density and attraction, the structure formation process remains unclear. Here, we show that the gelation of short-range attractive particles is governed by a nonequilibrium percolation process. We combine experiments on critical Casimir colloidal suspensions, numerical simulations, and analytical modeling with a master kinetic equation to show that cluster sizes and correlation lengths diverge with exponents  ~1.6 and 0.8, respectively, consistent with percolation theory, while detailed balance in the particle attachment and detachment processes is broken. Cluster masses exhibit power-law distributions with exponents  -3/2 and  -5/2 before and after percolation, as predicted by solutions to the master kinetic equation. These results revealing a nonequilibrium continuous phase transition unify the structural arrest and yielding into related frameworks.

Shear Thickening and Jamming of Dense Suspensions: The "Roll" of Friction.

Particle-based simulations of discontinuous shear thickening (DST) and shear jamming (SJ) suspensions are used to study the role of stress-activated constraints, with an emphasis on resistance to gearlike rolling. Rolling friction decreases the volume fraction required for DST and SJ, in quantitative agreement with real-life suspensions with adhesive surface chemistries and "rough" particle shapes. It sets a distinct structure of the frictional force network compared to only sliding friction, and from a dynamical perspective leads to an increase in the velocity correlation length, in part responsible for the increased viscosity. The physics of rolling friction is thus a key element in achieving a comprehensive understanding of strongly shear-thickening materials.

Tunable solidification of cornstarch under impact: How to make someone walking on cornstarch sink.

Hundreds of YouTube videos show people running on cornstarch suspensions demonstrating that dense shear thickening suspensions solidify under impact. Such processes are mimicked by impacting and pulling out a plate from the surface of a thickening cornstarch suspension. Here, using both experiments and simulations, we show that applying fast oscillatory shear transverse to the primary impact or extension directions tunes the degree of solidification. The forces acting on the impacting surface are modified by varying the dimensionless ratio of the orthogonal shear to the compression and extension flow rate. Simulations show varying this parameter changes the number of particle contacts governing solidification. To demonstrate this strategy in an untethered context, we show the sinking speed of a cylinder dropped onto the suspension varies markedly by changing this dimensionless ratio. These results suggest applying orthogonal shear while people are running on cornstarch would de-solidify the suspension and cause them to sink.

Stokes at 200: a celebration of the remarkable achievements of Sir George Gabriel Stokes two hundred years after his birth.

Sir George Gabriel Stokes PRS was for 30 years an inimitable Secretary of the Royal Society and its President from 1885 to 1890. Two hundred years after his birth, Stokes is a towering figure in physics and applied mathematics; fluids, asymptotics, optics, acoustics among many other fields. At the Stokes200 meeting, held at Pembroke College, Cambridge from 15-18th September 2019, an invited audience of about 100 discussed the state of the art in all the modern research fields that have sprung from his work in physics and mathematics, along with the history of how we have got from Stokes' contributions to where we are now. This theme issue is based on work presented at the Stokes200 meeting. In bringing together people whose work today is based upon Stokes' own, we aim to emphasize his influence and legacy at 200 to the community as a whole. This article is part of the theme issue 'Stokes at 200 (Part 1)'.

Absorbing-State Transitions in Granular Materials Close to Jamming.

We consider a model for driven particulate matter in which absorbing states can be reached both by particle isolation and by particle caging. The model predicts a nonequilibrium phase diagram in which analogs of hydrodynamic and elastic reversibility emerge at low and high volume fractions respectively, partially separated by a diffusive, nonabsorbing region. We thus find a single phase boundary that spans the onset of chaos in sheared suspensions to the onset of yielding in jammed packings. This boundary has the properties of a nonequilibrium second order phase transition, leading us to write a Manna-like mean field description that captures the model predictions. Dependent on contact details, jamming marks either a direct transition between the two absorbing states, or occurs within the diffusive region.

On the role of flexibility in linker-mediated DNA hydrogels.

Three-dimensional DNA networks, composed of tri- or higher valent nanostars with sticky, single-stranded DNA overhangs, have been previously studied in the context of designing thermally responsive, viscoelastic hydrogels. In this work, we use linker-mediated gels, where the sticky ends of two trivalent nanostars are connected through the complementary sticky ends of a linear DNA duplex. We can design this connection to be either rigid or flexible by introducing flexible, non-binding bases. The additional flexibility provided by these non-binding bases influences the effective elasticity of the percolating gel formed at low temperatures. Here we show that by choosing the right length of the linear duplex and non-binding flexible joints, we obtain a completely different phase behaviour to that observed for rigid linkers. In particular, we use dynamic light scattering as a microrheological tool to monitor the self-assembly of DNA nanostars with linear linkers as a function of temperature. While we observe classical gelation when using rigid linkers, the presence of flexible joints leads to a cluster fluid with a much-reduced viscosity. Using both the oxDNA model and a coarse-grained simulation to investigate the nanostar-linker topology, we hypothesise on the possible structure formed by the DNA clusters. Moreover, we present a systematic study of the strong viscosity increase of aqueous solutions in the presence of these DNA building blocks.

Testing the Wyart-Cates model for non-Brownian shear thickening using bidisperse suspensions.

There is a growing consensus that shear thickening of concentrated dispersions is driven by the formation of stress-induced frictional contacts. The Wyart-Cates (WC) model of this phenomenon, in which the microphysics of the contacts enters solely via the fraction f of contacts that are frictional, can successfully fit flow curves for suspensions of weakly polydisperse spheres. However, its validity for "real-life", polydisperse suspensions has yet to be seriously tested. By performing systematic simulations on bidisperse mixtures of spheres, we show that the WC model applies only in the monodisperse limit and fails when substantial bidispersity is introduced. We trace the failure of the model to its inability to distinguish large-large, large-small and small-small frictional contacts. By fitting our data using a polydisperse analogue of f that depends separately on the fraction of each of these contact types, we show that the WC picture of shear thickening is incomplete. Systematic experiments on model shear-thickening suspensions corroborate our findings, but highlight important challenges in rigorously testing the WC model with real systems. Our results prompt new questions about the microphysics of thickening for both monodisperse and polydisperse systems.

Expert Consensus on Growth and Development Curricula for Predoctoral and Advanced Education Orthodontic Programs.

Dental students and orthodontic residents must demonstrate competence in various areas prior to graduation. However, the Commission on Dental Accreditation (CODA) definition of competence is broad. The aims of this study were to obtain expert consensus on Growth and Development topics and subtopics in predoctoral and advanced education programs in orthodontics and to determine the level of cognition on the subtopics necessary to demonstrate learner competence. A modified Delphi method with a consensus threshold of 70% was used. In Round One, academic orthodontists who met the expert panel inclusion criteria were surveyed to determine whether a topic was necessary to the curricula. Round Two identified the subtopics under each topic and the level of cognition necessary to demonstrate learner competence using Bloom's taxonomy, which defines the ascending levels of remember, understand, apply, analyze, evaluate, and create. Round Three involved analyses and interpretation of Round Two results. The academic orthodontists determined that six topics (n=24) and 29 subtopics (n=15) were necessary for Growth and Development curricula. For all subtopics, they determined the mean level of cognition for predoctoral education was understand; for advanced education, it was analyze. This consensus on Growth and Development curricular content suggests that these levels are necessary for a beginning dentist and a beginning orthodontist to be deemed competent. Findings from this study can serve to guide curricular development and instruction by using the identified learning objectives to build instructional and assessment measures.

Parameter-free predictions of the viscoelastic response of glassy polymers from non-affine lattice dynamics.

We study the viscoelastic response of amorphous polymers using theory and simulations. By accounting for internal stresses and considering instantaneous normal modes (INMs) within athermal non-affine theory, we make parameter-free predictions of the dynamic viscoelastic moduli obtained in coarse-grained simulations of polymer glasses at non-zero temperatures. The theoretical results show very good correspondence with rheology data collected from molecular dynamics simulations over five orders of magnitude in frequency, with some instabilities that accumulate in the low-frequency part on approach to the glass transition. These results provide evidence that the mechanical glass transition itself is continuous and thus represents a crossover rather than a true phase transition. The relatively sharp drop of the low-frequency storage modulus across the glass transition temperature can be explained mechanistically within the proposed theory: the proliferation of low-eigenfrequency vibrational excitations (boson peak and nearly-zero energy excitations) is directly responsible for the rapid growth of a negative non-affine contribution to the storage modulus.

Shaken and stirred: Random organization reduces viscosity and dissipation in granular suspensions.

The viscosity of suspensions of large (≥10 µm) particles diverges at high solid fractions due to proliferation of frictional particle contacts. Reducing friction, to allow or improve flowability, is usually achieved by tuning the composition, either by changing particle sizes and shapes or by adding lubricating molecules. We present numerical simulations that demonstrate a complementary approach whereby the viscosity divergence is shifted by driven flow tuning, using superimposed shear oscillations in various configurations to facilitate a primary flow. The oscillations drive the suspension toward an out-of-equilibrium, absorbing state phase transition, where frictional particle contacts that dominate the viscosity are reduced in a self-organizing manner. The method can allow otherwise jammed states to flow; even for unjammed states, it can substantially decrease the energy dissipated per unit strain. This creates a practicable route to flow enhancement across a broad range of suspensions where compositional tuning is undesirable or problematic.